Localized aerodynamic drag is more pronounced on the perimeter of certain groups of aerodynamic bodies passing through air. In an ink jet array, the differential aerodynamic retardation between streams causes progressive drop velocity reduction from the center to either edge of the ink jet array, which results in misregistration of droplets on the printing medium. The cause of the retardation of the droplets near the edge of the printing medium relative to those near the center is caused by the aerodynamic drag. All droplets experience a velocity reduction during flight, but the effect is more pronounced near the array edges, where the droplets are less shielded from the ambient air. The consequence of aerodynamic retardation is that droplets near the array edge strike the printing medium at a later time than do droplets near the center. The printed array which results is a curved rather than a straight line as desired.
Two methods of correcting for the effect of aerodynamic retardation on a droplet array are set forth in an article entitled "Reducing Drop Misregistration From Differential Aerodynamic Retardation in a Linear Ink Jet Array" by D. E. Fisher and D. L. Sipple in the IBM Technical Disclosure Bulletin, Volume 17, No. 10, March 1975.
The first correction method compensates for the effect of aerodynamic retardation by applying deflection information to the outer droplets in the array at a predetermined time earlier than the deflection information is applied to the droplets near the center. This results in a curvature opposite to that caused by aerodynamic retardation, however, after a given amount of flight time, this initial curve tends to reverse itself, and if properly applied results in a straight line of droplets striking the printing medium. The use of control algorithms to vary the deflection information for the different streams is difficult to implement, and results in an increase in system hardware and cost.
The second method set forth for correcting the effects of aerodynamic retardation is to use a compensating velocity across the array rather than varying the time of flight. If the velocities of the edge streams are greater at break-off by a properly chosen amount than those on the interior, then aerodynamic retardation tends to negate the effect of the induced velocity nonuniformity during flight, causing the droplets to arrive at the paper simultaneously. The velocity variation at break-off can be accomplished by making the nozzle exit diameters progressively larger from the array center to both edges. This, however, requires precision tooling of the ink jet array head which is relatively expensive and difficult to implement.
In U.S. Pat. No. 3,562,757 of Bischoff, an ink jet system is disclosed having a single stream, wherein drop-to-drop retardation is corrected for, rather than a stream-to-stream differential retardation. Bischoff states that each drop in a stream leaves in its wake a region of turbulence which causes some amount of uncertainty in the predictable path of a following drop which enters the region of turbulence. Bischoff, therefore, charges every other drop, such that every other drop is guttered thereby affecting an increase in distance between the drops which are used for printing, thereby reducing the wake between the drops used for printing. The system set forth in Bischoff for correcting for drop-to-drop retardation would have little if any effect on compensating for the differential retardation from stream to stream in a linear ink jet array.
U.S. Pat. No. 3,596,275 of Sweet also deals with the problem of drop to drop retardation. Sweet, however, teaches the use of a colinear stream of air with the ink droplet stream to reduce the effects of wake of a given droplet relative to a following droplet. There is no teaching in Sweet of the use of such a colinear airstream to reduce the effects of stream-to-stream differential retardation, however, the colinear airstream would result in such a reduction of the differential aerodynamic retardation. The means to produce the colinear airstream, however, is difficult to achieve. This is so, since the passageway for the airflow must be properly designed to result in laminar airflow. Also, a fan or the like must be utilized to generate the airflow which increases the bulk and cost of the system.
According to the present invention, method and apparatus is set forth for reducing the effect of differential aerodynamic retardation between streams by utilizing droplet streams on the perimeter of the array which are continuously guttered. The continuously guttered droplet streams produce an airflow which tends to reduce the aerodynamic retardation of the droplet streams on the interior of the array which are used for printing. The use of continuously guttered droplet streams for producing an airflow is relatively inexpensive and easy to implement.